1. Field of the Invention
This invention relates to a method for grain equalization utilizing double dotting, comprising the steps of: determining a drop weight of each die in a multi-die printhead to print a given ink; determining the highest drop weight amongst all of the dies; and determining a percentage of double dotting that is needed to substantially match a grain of the highest drop weight die.
2. Description of the Related Art
Compared to thermal ink jet printing systems that employ only a single die for each primary color, multi-die architectures present unique challenges in the area of color consistency. For printers using a single die for a given ink, die-to-die drop weight differences can cause color shifts between print jobs, but the color remains consistent within each print, as shown in FIG. 1a. As can be seen in FIG. 1a, there is a color shift between print A and print B even though both prints were printed by the same type of print head but of different drop weights for the same ink color. In contrast, for multi-die systems, uncompensated die-to-die drop weight differences can cause bands of visible color differences within any given print job, as shown in FIG. 1b. Even small differences in drop weight between dies can become visible and reduce customer satisfaction.
It is known in the color compensation art to employ a color compensation algorithm. Using the color compensation algorithm, the relative printed dot densities, or numbers of dots per unit area, can effectively be adjusted at the individual die level. This compensates for natural drop weight variations caused by the manufacturing process, as well as colorant concentration differences and establishes better color consistency between prints generated from different dies of the same color.
For multiple die per color printing systems, while the color compensation algorithm can match colors between dies with different drop weights, grain differences can be observed between printed die regions when drop weight variation exceeds a threshold (approximately 15%). FIG. 2a illustrates printing an area fill with a high drop weight. FIG. 2b illustrates what happens with a low drop weight die when the color compensation algorithm is used for color matching. Even though the color between the high drop weight area fill and the low drop weight area fill may be similar, FIG. 2a has significantly higher grain in FIG. 2b. 
It is also known that trickle warming can be used to raise the temperature of low drop weight dies. Dies operated at a higher temperature have a higher drop weight. By setting the trickle warming temperatures of individual dies appropriately, equal drop weights can be established between them. However, the drop volume range in which trickle warming can be used is limited. Independent of drop weight, trickle warming is used to improve drop ink ejection performance and reliability by warming a die to a minimum temperature (i.e. 45° C. trickle warming set point). A high drop weight die cannot use a trickle warming temperature that is lower than a minimum trickle warming set point selected for drop ejection performance and reliability. Higher trickle warming temperatures can cause problems with material's reliability, ink out-gassing, and puddling. Conversely, a low drop weight die cannot use a trickle warming temperature that is higher than the maximum trickle warming set point. In short, the trickle warming approach can only be used over an allowed temperature range that limits the ability to adjust for larger drop volume variations between dies.
It is apparent from the above that there exists a need in the art for a color compensation method and apparatus that reduces graininess in multi-die inkjet printing systems through the use of double dotting and does not rely heavily upon the utilization of the trickle warming approach It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.